The hypothalamic peptide, gonadotropin-releasing hormone (GnRH), is the key neuroendocrine regulator of mammalian reproductive development and function. At the level of the anterior pituitary, GnRH binds to the GnRH receptor (GnRHR), located on the cell surface of pituitary gonadotropes, to stimulate the synthesis and release of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Levels of GnRHR in the gonadotrope are highly regulated, and the responses of gonadotropes to GnRH correlate directly with the concentration of GnRHR on the cell surface. Several hormones, most notably GnRH itself, are critical regulators of the GnRHR gene. GnRH is released in a pulsatile manner, with the frequency and amplitude of GnRH pulses varying temporally and developmentally. These patterns of pulsatile GnRH release lead to differential LH and FSH synthesis and secretion and are pivotal for the regulation of reproductive development and fertility. The mechanism by which the gonadotrope decodes GnRH pulse frequency to differentially control LH and FSH remains a major question in reproductive endocrinology. The long-term goal of this project is to identify the mechanisms underlying GnRH pulse frequency-dependent differential regulation of gonadotropin gene expression and thereby LH and FSH synthesis and secretion. The hypothesis to be tested is that cell surface levels of GnRHR, as determined by the pattern of pulsatile GnRH, are a critical component of the gonadotrope GnRH pulse frequency decoder, determining the signal transduction pathways and transcriptional networks activated by GnRH to ultimately dictate LH and FSH production. The specific objective of this proposal is to test this hypothesis by identifying the signal transduction pathways and transcriptional networks activated through the GnRHR at varying GnRH pulse frequencies in vitro and in vivo, and to determine the role of cell surface GnRHR in dictating these pathways. In the first aim, we will identify the signal transduction pathways activated by pulsatile GnRH to stimulate FSH at low GnRHR levels, which correspond to low GnRH pulse frequencies. In the second aim, we will identify the signal transduction pathways activated by pulsatile GnRH to stimulate LH, and to reduce FSH stimulation, at high GnRHR levels, which correspond to high GnRH pulse frequencies. In the third aim, we will determine the effects of dysregulation of GnRHR expression by site-specific GnRHR gene modification on reproductive function in vivo in a genetic mouse model. Finally, in the fourth aim, we will determine the effects of gonadotrope-specific deletion of Gs or Gq/11 on reproductive function in vivo in genetic mouse models. The successful completion of these aims will lead to a better understanding of the nature of the GnRH pulse frequency decoder and will advance our knowledge about the role of the GnRHR in reproductive physiology. Identification of these regulatory pathways will provide insight relevant to the treatment of clinical conditions including polycystic ovarian syndrome (PCOS), hypothalamic amenorrhea, disorders of pubertal maturation, and infertility.